Outbreaks of Insect Herbivores on Plants Along Motorways in the United Kingdom

Recent outbreaks of defoliating insects on trees and shrubs used for landscaping major roads in the U.K. are reported. The reasons for some outbreaks were investigated with special reference to effects of the roadside environment. An outbreak of Phalera bucephala on Fagus sylvatia was examined each year between 1975 and 1979 and one of Euproctis similis on Crataegus monogyna from 1975-1978. The nitrogen content of vegetation near a heavily used motorway was greatly enhanced, probably by oxides of nitrogen emitted from vehicle exhausts. The increased nitrogen content of the plants probably increases the insect populations. The outbreaks could not be explained by relaxation of predation.